The non-power user is typically able to pickup and use OOo and feel comfortable in about one week. The non-power user will generally perform simple non-advanced tasks.
The power user, however, usually requires closer to one month to figure out how to accomplish the "advanced" tasks in OOo rather than in MSO. As an advanced OOo user, I was asked how to accomplish a specific format in a text table in MSO. After 30 minutes, we gave up. I knew how to do this in OOo in seconds. MSO may have supported the effect, but neither of use could figure out how to make this work.
To address the needs of both, it is recommended to have some sort of documentation, and perhaps even some class time, to help during the transition. It helps a lot to have a few power users learn OOo first and get them on board. A few dissenters can railroad the entire effort. The usual recommendation is that MSO be completely removed so that hold-outs are not continuing to use MSO.
You should verify that at least the majority of your document's are usable/readable OOo. Having spent years moving documents between MSO and OOo, I have a handle on what is more likely to cause problems (at least for text type documents). A typical problem is related to graphics that is not anchored as a character and that is free to float around the page (this is the default use).
For power point / presentation documents, there are some effects that may not translate well.
Macro compatibility is not good between MSO and OOo.